The operating window for low-density polyethylene (LDPE) extrusion coating: adhesion, line speed and substrate choice is established by the molten film’s oxidative residence time and the substrate’s surface character. The coating resin is a high-pressure LDPE homopolymer with density 0.917–0.924 g/cm³ per ISO 1183-1:2019 and melt mass-flow rate 4–15 g/10 min at 190°C/2.16 kg per ISO 1133-1:2022. The extrusion line consists of a single-screw extruder with 30:1–33:1 L/D and a barrier or Maddock mixing section, a flat die with internal deckle, an air gap, a chrome-plated chill roll, and a rubber-covered nip roll. Barrel temperatures rise from 180°C at the feed section to 315–335°C at the die and adapter. The die gap is typically 0.5–0.8 mm, and the air gap is 150–300 mm. In this gap, atmospheric oxidation generates polar carbonyl, hydroxyl, and carboxyl species on the molten web; these groups are the primary adhesion mechanism on non-porous substrates. Chill roll temperatures of 10–20°C and nip pressure between 25 N/mm and 75 N/mm of web width complete the quench and pressing sequence.

Because melt residence time in the air gap equals gap length divided by line speed, increasing line speed from 100 m/min to 300 m/min at a 200 mm air gap reduces oxidation exposure from 0.12 s to 0.04 s. This dependency makes line speed an adhesion variable, not merely a productivity variable.

How Does Melt Oxidation and Air-Gap Residence Time Control Adhesion to Porous and Non-Porous Substrates?

On kraft paper and uncoated paperboard, adhesion is partly mechanical. The low-viscosity melt penetrates surface fibres and locks during quenching. For paper substrates with Bendtsen roughness above 250 mL/min per ISO 8791-2:2013 and Cobb water absorption between 20 g/m² and 80 g/m² per ISO 535:2023, fibre tear adhesion can usually be achieved without primers. The melt oxidation layer still contributes because carbonyl radicals can hydrogen-bond to cellulose hydroxyl groups. On aluminium foil and biaxially oriented PET, mechanical interlocking is negligible. Adhesion depends on the concentration of polar oxygen species generated during the air gap. Oxidation is controlled by melt temperature, air gap length, and line speed. A temperature increase from 315°C to 335°C raises the oxidation rate and improves foil peel strength but also accelerates gel formation, odour, and smoke. At air gaps below 100 mm, oxidation may be insufficient for non-porous webs; above 300 mm, the melt curtain sags, neck-in increases, and edge trim rises.

Ozone treatment of the melt surface at 0.5–3.0 kW/m of web width or corona discharge at 38–50 dyn/cm on the substrate adds polar functionality without additional thermal load. For aluminium foil, the standard primer route uses EAA copolymer applied at 0.02–0.10 g/m² dry coat weight to provide carboxylic acid sites. Adhesion to foil is commonly measured by T-peel after conditioning at 23°C/50% RH per ASTM D1876-08(2023). Typical foil adhesion with EAA primer or ozone treatment ranges from 250 g/25 mm to 700 g/25 mm, depending on foil gauge and primer coat weight. ISO 8510-1:2008 may also be used for 90° peel of flexible laminates. On paper, the adhesion criterion is often fibre tear percentage rather than peel force; production specifications may require ≥90% fibre tear over the coated width.

Line speed in LDPE extrusion coating is not an independent variable because coat weight, melt curtain stability, and web thermal load are all speed-dependent. For a fixed die output rate, coat weight decreases as the inverse of line speed and web width: Coat weight (g/m²) = [melt output (kg/h) × 1000] / [web width (m) × line speed (m/h)]. On liquid packaging board, line speeds of 150–400 m/min are common with coat weights of 10–35 g/m². On aluminium foil from 6 µm to 20 µm, line speed is usually limited to 80–200 m/min because of foil brittleness and thermal expansion. On thin BOPP and BOPET films, line speed is constrained below 150 m/min by web distortion and heat shrinkage unless a chill roll with 10–15°C water is used.

Neck-in—the reduction in coated width relative to die width—is driven by melt elasticity and line speed. A 4–7 g/10 min LDPE may exhibit neck-in of 20–40 mm per side at 200 m/min, while a 10–15 g/10 min grade may exceed 50 mm per side. Draw resonance appears as a sustained oscillation in coating thickness when the draw ratio exceeds a critical value. For LDPE extrusion coating, critical draw ratio at 320°C melt temperature is typically between 30:1 and 100:1; higher melt strength grades extend it. The air gap must be increased for high line speed, but oxidation then becomes harder to control. Melt curtain tension is measured by load cells on the stripper/pressure assembly; at speeds above 250 m/min, pressure roll runout must be below 0.01 mm TIR to prevent chatter bands. Edge bead trim width increases with neck-in, and trim rates above 10% of die width indicate that the grade or deckle setting is unsuitable for the requested speed.

Adhesion Promoters, Primer Coatings, and Surface Activation Limits on Non-Porous Webs

Substrate choice determines whether LDPE adhesion is mechanical, chemical, or polar. For aluminium foil, the standard practice is to apply an EAA copolymer primer at 0.02–0.10 g/m² dry coat weight before extrusion. EAA provides carboxylic acid sites that react with aluminium oxide and diffuse into the LDPE melt. For PET and BOPP, corona treatment to 42–48 dyn/cm creates hydroxyl and carboxyl groups but the effect decays within hours; in-line corona is used immediately before the nip. For woven polypropylene and oriented polypropylene, a water-based polyethylenimine primer or a thin coextruded tie layer of EAA or ethylene vinyl acetate is often required. Without these, LDPE peel adhesion can be below 50 g/25 mm and fails cohesively at the interface. With primers, T-peel adhesion on PET may reach 200–400 g/25 mm per ASTM D1876-08(2023). For food-contact applications, the LDPE must meet FDA 21 CFR 177.1520(a)(2) and EU Regulation No 10/2011 overall migration limit of 10 mg/dm²; primers and corona by-products must also be compliant.

Substrate moisture and surface contamination impose tighter boundaries than often recognised. Paper and paperboard with moisture content outside 5–8% generate steam during coating, producing pinholes and reducing adhesion. Clay-coated boards with high binder content can delaminate under high nip pressure; the failure occurs within the paper coating rather than at the LDPE interface. Aluminium foil with residual rolling oil lowers surface energy below 34 dyn/cm; corona or flame treatment before priming is required. For BOPP and PET, uncontrolled slip additives can migrate to the surface and reduce adhesion after 48–72 h. The extrusion coating line should therefore include in-line surface tension measurement and a primer station with gravimetric coat-weight control. When line speed is increased, surface treatment residence time decreases; corona output must be raised to maintain the same dyne level, but overtreatment can embrittle films and create reverse-side printability loss.

Coextrusion of LDPE with an acid copolymer tie layer modifies adhesion on non-porous substrates without a primer station. A two-layer die with EAA at 5–15% of total coat weight can replace liquid primers on foil and BOPET. The LDPE skin layer still needs oxidation for sealability and surface chemistry. Coextrusion increases line complexity and requires separate extruders with dedicated temperature profiles; the tie layer must be extruded at 260–300°C to avoid degradation while the LDPE skin is maintained at 315–335°C. This configuration is preferred when solventborne primers are restricted by REACH or food-contact compliance requirements.

If Substrate Moisture or Surface Energy Falls Outside Narrow Bounds, Coating Defects Multiply

Production-scale failure modes observed on extrusion coating lines include edge bead tearing at slitter stations from excessive neck-in, melt curtain vibration from improper deckle settings, and delamination on foil caused by insufficient primer wetting. A sudden increase in line speed without increasing melt temperature reduces adhesion on paper but may improve it on foil if the shorter air gap is compensated by ozone. The safe operating envelope for a 7 g/10 min LDPE on kraft paper at 20 g/m² coat weight is often bounded by 150–350 m/min; below 150 m/min, oxidative exposure becomes excessive and off-taste compounds increase; above 350 m/min, pinhole count per m² rises because the melt curtain becomes unstable. On 9 µm aluminium foil, the practical upper speed is lower, near 120 m/min, due to foil handling and adhesion requirements. Published data for specific combinations of substrate grade, primer, and coat weight is limited; therefore, confirmation runs on the actual line are required before setting peel-adhesion specifications below 200 g/25 mm.